Regulation of denitrification in organic riparian soils

Abstract

Riparian zones have been shown to remove nitrate from groundwater before it enters surface waters thus providing protection of surface water quality. Microbial denitrification is considered to be a mechanism which may be responsible for nitrate removal in riparian zones. The aim of the present study was to identify the rate at which denitrification occurred in riparian zones and to identify factors which regulated the rate. This was investigated using laboratory and field studies. In laboratory studies, the addition of a range of single sources of carbon promoted denitrification in organic riparian soil during 10-day incubations. Denitrification lag times and rates were shown to be dependent on the broad groups of organic compounds tested. The denitrifier population responded most rapidly following the addition of a range of sugars, whereas the denitrification rate was highest following the addition of the fermentation endproducts lactate and ethanol. Cellulose and propionate did not stimulate denitrification above the control. Denitrification showed a diauxic response to xylan addition. Volatile fatty acids (VFAs) were shown to inhibit denitrification for at least the first 100 hours of incubation, after which acetate and butyrate were used by denitrifiers: however, propionate continued to be inhibitory. It was thought that VFAs inhibited bacteria following acidification of the bacterial cytoplasm. A longer-term incubation experiment (100 days) examined the suitability of three plant materials (fresh pine needles, senescent pine needles, and fresh watercress leaves) undergoing decomposition to act as electron donors for denitrification. Although denitrification was stimulated following the addition of all plant materials denitrification was greatest in those incubations where fresh rather than senescent plant material was added. Nitrous oxide, methane, and carbon dioxide production were shown to be dependent on the plant substrate added. Results demonstrated the interactive ecology of denitrifying, methanogenic, and fermentative bacteria in these anaerobic soils. Evidence suggested that denitrifying bacteria were dependent on fermentative bacteria making the added plant carbon more readily available. The long-term persistence (>100 days) of denitrifying activity in the absence of electron acceptors was also demonstrated. Field studies were conducted in an organic riparian zone receiving nitrate from incoming groundwater. Mean and median denitrification rates were found to be 1.2 and 0.98 gN.m⁻².day⁻¹; while mean and median N₂O production rates were found to be 0.073 and 0.088 gN.m⁻².day⁻¹. These rates were 1 to 3 orders of magnitude greater than those reported in previous studies of upland soils. Up to 77% of the variation in onsite denitrification rate could be explained by nitrate concentration and denitrifying enzyme activity. Temperature may also have regulated the rate at which denitrification occurred; however, too few sampling trips at different temperatures were conducted to fully establish a temperature effect. Nitrate concentrations, and consequently denitrification rates, were highest at the upslope edge of the riparian zone and decreased towards the downslope end. The denitrification rate appeared to be high enough to be responsible for the major proportion of nitrate removal observed in this riparian zone.